How Old Are You from How-old.net?

Hi guys, can the site how-old.net give someon's accurate age? I am asking this because I have tried uploading my photo but it kept on giving an error message that my face was not detected. Does it mean therefore that howlold.net only can I dentify one's age by the face? Why is it so? I am asking this because in medical and even general biology states that growth does not occur in only one part of the body but all parts grow though at different rates. If this argument olds, then therefore, the application how-old.net may be invalid in it's method of determining somebody's age. What are your thoughts?